310 PRINCIPLES OF CHEMISTRY 



It has been shown above that if manganese dioxide, or any 

 lower oxide of manganese, be heated with an alkali in the presence of 

 air, the mixture absorbs oxygen, 21 and forms an alkaline manganate of a 

 green colour: 2KHO + Mn0 2 4- O = K 2 Mn0 4 + H 2 0. Steam is disen- 

 gaged during the ignition of the mixture, and if this does not take 

 place there is no absorption of oxygen. The oxidation proceeds much 

 more rapidly if, before igniting in air, potassium chlorate or nitre be 

 added to the mixture, and this is the method of preparing potassium 

 manganate, K 2 MnO 4 . The resultant mass dissolved in a small quantity 

 of water gives a dark green solution, which, when evaporated under the 

 receiver of an air pump over sulphuric acid, deposits green crystals of 

 exactly the same form as potassium sulphate namely, six-sided prisms 

 and pyramids. The composition of the product is not changed by being 

 redissolved, if perfectly pure water free from air and carbonic acid be 

 taken. But in the presence of even very feeble acids the solution of 

 this salt changes its colour and becomes red, and deposits manganese 

 dioxide. The same decomposition takes place when the salt is heated 

 with water, but when diluted with a large quantity of unboiled water 

 manganese dioxide does not separate, although the solution turns red, 

 This change of colour depends on the fact that potassium manganate, 

 K 2 MnO 4 , whose solution is green, is transformed into potassium per- 

 manganate, KMnO 4 , whose solution is of a red colour. The reaction 

 proceeding under the influence of acids and a large quantity of water 



furnace, obtained carbides of manganese for example, Mn 3 C and remarked that the metal 

 volatilised in the heat of the voltaic arc. Metallic manganese is, however, not prepared on' 

 a large scale, but only its alloys with carbon (they readily and rapidly oxidise) and ferro- 

 manganese or a coarsely crystalline alloy of iron, manganese and carbon, which is 

 smelted in blast-furnaces like pig-iron (see Chapter XXII.) This ferro-manganese is 

 employed in the manufacture of steel by Bessemer's and other processes (see Chapter 

 XXII.) and for the manufacture of manganese bronze. However, in America, Green and 

 Wahl (1895) obtained almost pure metallic manganese on a large scale. They first treat 

 the ore of MnO. 3 with 30 p.c. sulphuric acid (which extracts all the oxides of iron 

 present in the ore), and then Iveat it in a reducing flame to convert it into MnO, which 

 they mix with a powder of Al, lime and CaFo (as a flux), and heat the mixture in a 

 crucible lined with magnesia ; a reaction immediately takes place at a certain temperature, 

 and a metal of specific gravity 7'3 is obtained, which only contains a small trace of iron. 



Manganese gives two compounds with nitrogen, Mn 5 N 2 and Mn^No. They were 

 obtained by Prelinger (1894) from the amalgam of manganese Mn ? Hg 5 (obtained on a 

 mercury anode by the action of an electric current upon a solution of MnCh) ; the 

 mercury may be removed from this amalgam by heating it in an atmosphere of hydrogen, 

 and then metallic manganese is obtained as a grey porous mass of specific gravity 7'42. 

 If this amalgam be heated in dry nitrogen it gives Mn 5 N 2 (grey powder, sp. gr. 6'58), but 

 if heated in an atmosphere of NH 3 it gives (as also does Mn 5 N 2 ) Mn 5 N 2 , (a dark mass 

 with a metallic lustre, sp. gr. G - 21), which, when heated in nitrogen is converted into 

 Mn 5 N.;, and if heated in hydrogen evolves NH 5 and disengages hydrogen from a solution 

 of NH 4 C1. At all events, manganese is a metal which decomposes water more easily 

 than iron, nickel, and cobalt. 



Volume I. p. 157. Note 7. 



